EP1207970A2 - Electrostatic ultrasonic transducer and method of making the same - Google Patents
Electrostatic ultrasonic transducer and method of making the sameInfo
- Publication number
- EP1207970A2 EP1207970A2 EP00930807A EP00930807A EP1207970A2 EP 1207970 A2 EP1207970 A2 EP 1207970A2 EP 00930807 A EP00930807 A EP 00930807A EP 00930807 A EP00930807 A EP 00930807A EP 1207970 A2 EP1207970 A2 EP 1207970A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- transducer
- insulating film
- electrodes
- electrode
- layer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H11/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties
- G01H11/06—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties by electric means
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
- B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
- B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
- B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
- B06B1/0292—Electrostatic transducers, e.g. electret-type
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/42—Piezoelectric device making
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/49005—Acoustic transducer
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/49007—Indicating transducer
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/4902—Electromagnet, transformer or inductor
- Y10T29/4908—Acoustic transducer
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/49117—Conductor or circuit manufacturing
- Y10T29/49124—On flat or curved insulated base, e.g., printed circuit, etc.
- Y10T29/49155—Manufacturing circuit on or in base
- Y10T29/49156—Manufacturing circuit on or in base with selective destruction of conductive paths
Definitions
- the present invention relates to the field of acoustic transducers, and more specifically to a novel electrostatic ultrasonic transducer capable of operating in high frequency ranges, and novel methods of fabricating such a transducer.
- An acoustic transducer is an electronic device used to emit and receive sound waves.
- An ultrasonic transducer is a type of acoustic transducer that operates at a frequency range beyond that of human perception, about 20 KHz.
- Acoustic transducers are used in medical imaging, non-destructive evaluation, and other applications.
- the most common forms of acoustic transducers are piezoelectric transducers, which operate m low and narrow band frequencies. Piezoelectric transducers are not efficient in the conversion between electric and acoustic energy in air. Furthermore, the operating frequencies of piezoelectric transducers in air are quite low. Air coupled ultrasonic transducers with higher operating frequencies, which rely on certain microfab ⁇ cation techniques, are described by Haller et al.
- the transducer disclosed therein is made of a substrate 11 and a gold contact layer 14 that forms one plate of a capacitor, and a membrane including a nitride layer 13 and a gold contact layer 14B that form the other plate of the capacitor (while the gold contact layer 14 is the electrode, with the nitride layer 13 being an insulator, the reference to electrode 13/14 will be used so as to distinguish the other electrode 11/14 that has a gold contact layer 14 adjacent the conductive substrate 11 as illustrated in the above-mentioned patents).
- Holes 16 etched in the nitride layer 13 and the gold layer 14 are used to etch away portions of the sacrificial oxide layer 12, while remaining posts of the sacrificial layer 12 support the membrane. By noting the change in capacitance between the two electrodes 13/14 and 11/14, the ultrasonic resonance of the membrane can be detected.
- micro fabricated ultrasonic transducers use resilient membranes that have very little inertia The momentum carried by approximately half of a wavelength of air molecules is able to set the membrane in motion and visa versa. Electrostatic actuation and detection enable the realization and control of such resonant membranes When distances are small, electrostatic attractions can exert very large forces on the actuators of interest. Microfabricated ultrasonic transducers of this design have practical problems that prohibit their use at high frequencies, typically above about 10MHz, and that reduce their efficiency at any frequency range. It has been realized by the present inventor that there are various reasons that prohibit the use of microfabricated ultrasonic transducers. One reason is that the electrodes 13/14 and 1 1/14 are each formed as a conductive sheet. As illustrated in FIG, 1A, while the gold contact layer
- the gold contact layer 14 covers the voids where the sacrificial layer 12 has been etched away, the gold contact layer 14 also entirely covers the posts which support the membrane. Similarly, the substrate 1 1 and the gold contact layer 14 associated therewith is another conductive sheet. Accordingly, at areas other than where sacrificial etch access holes 15 exist, there is no area where the electrodes 13/14 and 11/14 do not overlap. This overlap causes a parasitic capacitance, which is exacerbated due to the fact that the dielectric constant of the semiconductor insulators between the areas of the sacrificial layer 12 posts can be approximately one order of magnitude larger than that of the air/vacuum gap at the center of the membrane. As frequencies become higher, the parasitic capacitance becomes significant and sometimes even a dominant factor in transducer performance. Thus, even if the overlap at the areas of the sacrificial layer 12 posts accounts for only 1/10 of the active area of the transducer, such overlap may account for half the capacitance.
- the spacing between the top electrode 13/14 and the bottom electrode 11/14 is a further reason that the parasitic capacitance increases.
- the membrane has a thickness that, due to physical constraints, needs to be at least about 2,500 Angstroms thick.
- transducers such as those described in Haller et al or Ladabaum et al are not able to operate at higher frequencies, and operate less efficiently than ultimately possible at lower frequencies. Accordingly, there is the need for an improved acoustic transducer.
- the present invention achieves the above objects, among others, with an ultrasonic transducer comprised of a plurality of transducer cells, and conductive interconnects between the cells.
- Each transducer cell contains a bottom electrode formed on a layer of insulator material, a lower insulating film portion formed over the bottom electrode, a middle insulating film portion that includes an air/vacuum void region, and an upper insulating film portion that includes a top electrode formed within a portion of the upper insulating film portion.
- a first layer of interconnects electrically connect the bottom electrodes of each transducer cell and a second layer of interconnects electrically connect the top electrodes of each transducer cell.
- the top and bottom layers of interconnects are patterned to avoid overlap between them, thus reducing the parasitic capacitance.
- the top electrode is preferably formed within the upper insulating film portion, closer to the air/vacuum void than to the top surface of the insulating film, to increase the electric field for a given voltage. Still furthermore, the electrodes within each transducer cell are preferably formed to have dimensions that are smaller than the overall surface area of the insulating film that they excite.
- a method of fabricating the ultrasonic transducer according to the invention is initiated by depositing and forming a pattern of the bottom electrode and interconnects. Thereafter, the lower insulating film portion of insulator material is deposited. A sacrificial layer is then deposited over the lower insulating film portion and etched to a desired pattern. The middle insulating film portion of insulator material is deposited over the sacrificial layer pattern, followed by the depositing and forming of the top layer of electrode and interconnects. Thereafter, an upper insulating film portion of insulator material is deposited to complete the formation of the insulating film. Thereafter, the insulating film is etched to form a via hole that allows an etchant to reach the sacrificial layer pattern. Etching is then performed to remove the remaining sacrificial layer pattern to form void areas.
- the top layer of electrode and interconnects is formed so that the top layer interconnects do not overlap the bottom layer interconnects, thus reducing the parasitic capacitance.
- the ultrasonic transducer is comprised of a number of interconnected transducer cells. The transducer cells are electrically connected together to form a single ultrasonic transducer. Multiple transducers can be formed on the same substrate in an array. The ultrasonic transducers, and each of the transducer cells formed therein, are formed at the same time using the fabrication steps described above. Brief Description of the Drawings
- FIGS. 1A and IB illustrate a top-view and a cross-section of an electrostatic transducer as is known in the prior art
- FIG. 2 is a top-view of an electrostatic transducer according to a preferred embodiment of the invention.
- FIG. 3 is a cross-sectional view of an electrostatic transducer according to a preferred embodiment of the invention.
- FIGS. 4-15 illustrate a method of fabricating an electrostatic transducer according to a preferred embodiment of the invention.
- FIG. 2 illustrates a top-view diagram illustrating certain aspects of the present invention.
- a transducer 100 is illustrated as including three connected octagonal-shaped transducer cells 200A-C are shown.
- the transducer 100 may have as few as one or many more than three, such as hundreds or thousands, transducer cells 200 associated with it.
- Many such transducers 100 will typically be formed at the same time on a wafer, with the wafer cut into different die as is known in the art. The discussions hereinafter, however, will be made with respect to a single transducer 100.
- transducer cells 200 illustrated in FIG. 2 is for illustrative purposes, and it is understood that the shape of the transducer cell can be a variety of different shapes, such as hexagonal, round, square, rectangular, triangular, or any other suitable configuration.
- transducer cells 200 may be of different sizes to provide broadband frequency response.
- Transducer cells 200 may also be of certain shapes, such as rectangular, so that they may resonate at a plurality of frequencies. Any number of transducer cells 200 can be interconnected, as described in further detail hereinafter, to form a single transducer.
- top and bottom interconnects 220 and 230 that are used to electrically connect top and bottom electrodes, respectively, of adjacent transducer cells 200.
- the present invention forms transducer cells which each have their own top and bottom electrodes, and then interconnects having dimensions smaller than the entire sheet of conductor are used to electrically connect different electrodes.
- the top and bottom electrodes of the various cell transducers can be electrically viewed as being single top and bottom electrodes, the discussion hereinafter will use the top and bottom electrodes to refer to the electrodes associated with a single transducer cell.
- the interconnects when viewed from a top view, the interconnects
- a multi-membrane transducer is formed by interconnecting transducer cells 200A-C on a substrate 300.
- the transducer cells 200 may be of the same size, as shown, or be of different sizes.
- each transducer cell 200 is electrically connected to other transducer cells 200, such that each transducer cell 200 has a top electrode 350 linked by a top layer interconnect 220, as illustrated in FIG. 2.
- Each bottom electrode 320 (not shown in FIG. 2) is connected by a bottom layer interconnect 230. Accordingly, overlap of the interconnect 220 and 230 is avoided.
- the dielectric constant of semiconductor insulators such as nitride
- the dielectric constant of semiconductor insulators can be approximately one order of magnitude larger than that of the air/vacuum, by eliminating, or at least minimizing, overlap of the electrode interconnects, parasitic capacitance is reduced.
- FIG. 3 illustrates a cross-section taken along line 3-3 of Fig 2 of the plurality of transducer cells 200A-C.
- Each transducer cell 200 is contains an air/vacuum cavity 340 surrounded by an insulative insulating film layer 330, with a bottom electrode 320 and a top electrode 350 associated with each transducer cell 200.
- FIG. 3 illustrates a cross-section taken along line 3-3 of Fig 2 of the plurality of transducer cells 200A-C.
- Each transducer cell 200 is contains an air/vacuum cavity 340 surrounded by an insulative insulating film layer 330, with a bottom electrode 320 and a top electrode 350 associated with each transducer cell 200.
- FIG. 3 illustrates a cross-section taken along line 3-3 of Fig 2 of the plurality of transducer cells 200A-C.
- Each transducer cell 200 is contains an air/vacuum cavity 340 surrounded by an insulative insulating film layer 330, with a bottom electrode
- the dimension R is the thickness of the membrane, which is formed of a portion of the insulating layer 330 and the top electrode disposed therein, that is disposed above the air/vacuum cavity 340 and that is required for a certain acoustic impedance of the transducer cells, such impedance governing the frequency range of the transducer.
- FIG. 2 illustrates a certain known microfabricated electronic transducer that uses a gold contact layer 14 fabricated on the top surface of the nitride layer 13 as a top electrode of the transducer.
- the nitride layer 13 illustrated in FIG. 2 and the membrane of the present invention both must operate in the frequency range of interest, as described previously. In contrast to the structure illustrated in FIG.
- the present invention forms the top electrode 350 within the membrane. Accordingly, whereas the separation distance of the bottom and the top electrodes is D in the prior art electronic transducer described in FIG. 2, the separation distance of the bottom and top electrodes according to the present invention is S. Since the top electrode is formed within the membrane, the distance S will, for an otherwise equivalent transducer, always be less than D. By forming the top electrode within the membrane, as described fully hereinafter, parasitic capacitance in the present invention is further reduced.
- FIGS. 2 and 3 Another aspect of the invention illustrated by FIGS. 2 and 3 is that the surface area of the electrodes 320 and 350 is smaller than the surface area of the corresponding air/vacuum cavity 340. As noted hereinafter, this further allows for a reduction in the parasitic capacitance of the resulting ultrasonic transducer.
- the transducer cells 200 according to the present invention can have a variety of shapes and dimensions.
- membrane will typically have an area that ranges from about 300 to 30,000 ⁇ m " with a membrane thickness that ranges from about 0.05 to 1 ⁇ m, a residual stress in the PECVD nitride ranging from about 10 to 400 MPa and a gap thickness ranging from about 0.1 to 2 ⁇ m. It is understood, however, that these dimensions are illustrative only and that any dimensions which meet the characteristics of the invention described herein can be used, as previously mentioned.
- FIGS 4-15 The process of fabricating an acoustic transducer in accordance with a preferred embodiment of the invention will now be described with reference to FIGS 4-15. It will be apparent that various different steps and sequences of steps can be used to fabricate the acoustic transducer according to the invention. Starting with FIG. 4, the process begins with a silicon or other semiconductor support substrate
- a layer of thermal oxide 310 is grown, preferably having a thickness in the range of
- this conductor is aluminum (Al), but the conductor could also be any conductor known in the art, such as copper (Cu) or tungsten
- a resist pattern is transferred lithographically to the substrate, and the conductor 320 is etched to leave behind a patterned bottom electrodes 350 and associated interconnects.
- FIG. 5 illustrates the resultant patterned bottom electrodes 350A-C and Fig 2 illustrates the resultant patterned bottom interconnect.
- a lower insulating film portion 330A of the insulating film 330 is deposited.
- This lower insulating film portion 330A is an insulator, such as nitride, applied using, for instance a plasma-enhanced chemical vapor deposition (also known as "PECVD").
- PECVD plasma-enhanced chemical vapor deposition
- the applied lower insulating film portion 330A will typically have a measured residual stress that is less than 50 MPas. The residual stress may be adjusted by varying the frequency of the plasma and the relative concentration of nitrogen and silicon carrying gases.
- the lower insulating film portion 330A will typically be deposited to a thickness of about 0.25 ⁇ m.
- a sacrificial layer 700 as known in the art, such as aluminum or low temperature oxide (LTO), is deposited.
- the deposit thickness may range from 0.05 to 1 ⁇ m.
- a resist pattern is transferred lithographically, and the sacrificial layer 700 is etched to leave behind a pattern, such as shown in FIG. 8.
- the sacrificial layer contains portions 700A, 700B and 700C, which will each correspond to a void region that will be made within each transducer cell 200A, 200B and 200C, respectively.
- a pathway 702 which pathway 702 will allow for the etchant that removes the sacrificial layer to be introduced from a location that is physically separate from the transducer cells.
- a middle insulating film portion 330B is then deposited, preferably an insulator that is the same as that of the lower insulating film portion 330A.
- PECVD silicon nitride is deposited as the middle insulating film portion 330B to a thickness of about 0.15 ⁇ m over the patterned sacrificial layer 700 to surround and cover the patterned sacrificial layer 700, as illustrated by FIG. 9.
- the top conductor layer 920 is deposited, and subsequently etched in a pattern to produce a top electrode 350 and the resulting interconnects, as shown in FIGS. 3 and 1 1 and described previously.
- the electrodes 320A, 320B and 320C will overlap the electrodes 350A, 350B and 350C, respectively, the top interconnects 220 will not overlap with the bottom interconnects 230, as described previously. This is ensured by selection of an appropriate pattern for the top and bottom interconnects, one such pattern being illustrated in FIG. 2.
- top insulating film portion 330C of the insulating film 330 is then deposited, as shown in Fig 12, and the material for the top insulating film portion 330C is preferably the same as that used for the bottom insulating film portion 330A and the middle insulating film portion 330B, previously described.
- via holes 900 are created to provide for an etchant path to the remaining portions of the sacrificial layer, such as portions 700A, 700B, 700C and 702 illustrated in FIG. 8. Accordingly, after the via holes 900 are formed, the remaining portions of the sacrificial layer are then etched away by a sacrificial wet etch or other technique known in the art. For example, buffered hydrofluoric acid can be used in the case of a low temperature oxide (LTO) sacrificial layer 700.
- LTO low temperature oxide
- the sacrificial etch results in an air/vacuum cavities being formed, such as the cavities 340A, 340B and 340C illustrated in FIG. 14.
- the via holes 900 can be filled in, preferably using the same material as the insulating film 330, if needed, such as for an immersion transducer.
- the additional material added over the top insulating film portion 330C can also become part of the insulating film 330, or it can be subsequently etched from all areas except for the sealing locations.
Abstract
Description
Claims
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/315,896 US6271620B1 (en) | 1999-05-20 | 1999-05-20 | Acoustic transducer and method of making the same |
US315896 | 1999-05-20 | ||
PCT/US2000/013634 WO2000072631A2 (en) | 1999-05-20 | 2000-05-17 | Electrostatic ultrasonic transducer and method of making the same |
Publications (2)
Publication Number | Publication Date |
---|---|
EP1207970A2 true EP1207970A2 (en) | 2002-05-29 |
EP1207970B1 EP1207970B1 (en) | 2008-07-09 |
Family
ID=23226538
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP00930807A Expired - Lifetime EP1207970B1 (en) | 1999-05-20 | 2000-05-17 | Electrostatic ultrasonic transducer and method of making the same |
Country Status (6)
Country | Link |
---|---|
US (3) | US6271620B1 (en) |
EP (1) | EP1207970B1 (en) |
JP (1) | JP4519328B2 (en) |
AU (1) | AU4856500A (en) |
DE (1) | DE60039438D1 (en) |
WO (1) | WO2000072631A2 (en) |
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- 2000-05-17 DE DE60039438T patent/DE60039438D1/en not_active Expired - Lifetime
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AU4856500A (en) | 2000-12-12 |
JP4519328B2 (en) | 2010-08-04 |
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WO2000072631A3 (en) | 2001-05-03 |
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US20010043028A1 (en) | 2001-11-22 |
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US6571445B2 (en) | 2003-06-03 |
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